Method and system for production of fibrous composite prototypes using acoustic manipulation in stereolithography

ABSTRACT

A method for producing a three-dimensional object by stereolithography. A solid reinforcing material is mixed with the fluid medium so that at least a part of said solid reinforcing medium is located in the layer of the fluid medium between the top surface of the most recently formed lamina and the top surface of the fluid medium. An acoustic field is then established in the fluid medium such that this acoustic field exists in at least part of the layer of the fluid medium between the top surface of the most recently formed lamina and the top surface of the fluid medium. The solid reinforcing material is thereby moved with said acoustic force field. A three-dimensional reinforced object is thereby produced.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to stereolithography methods and systemsinvolving the application of lithographic techniques tothree-dimensional objects, and more particularly to providing structuralreinforcement of such three-dimensional objects.

(2) Brief Description of the Prior Art

Stereolithography is a “printing” process invented by Charles Hull in1986 by which three-dimensional copies of solid models are fabricated inplastic. This process is disclosed in U.S. Pat. No. 4,575,330 to Hull,the contents of which are incorporated herein by reference. The Hullpatent discloses a system for generating three-dimensional objects bycreating a cross sectional pattern of the object to be formed at aselected surface of a fluid medium. This fluid medium is capable ofaltering its physical state in response to appropriate synergisticstimulation by impinging radiation, particle bombardment or chemicalreaction. Successive adjacent laminae, representing correspondingsuccessive adjacent cross sections of the object, are automaticallyformed and integrated together to provide a step-wise laminar buildup ofthe desired object. A three-dimensional object is thereby formed anddrawn from a substantially planar surface of the fluid medium during theforming process. This process was the first solid imaging process thatallowed the fabrication of highly complex physical parts directly fromcomputer generated topology data as is disclosed by Jacobs in RapidPrototyping and Manufacturing: Fundamentals of StereoLithography (1992).

In fact, the advantages of stereolithography prototyping overtraditional machining become even more prominent with increasing partcomplexity. For example, parts involving intricate internal cavities orencased subparts that are impossible to machine as one part are easilyfabricated with stereolithography. Physical application of thestereolithography printing process for rapid prototyping takes place viaa commercial system known as a stereolithography apparatus (SLA),manufactured by 3D Systems, Inc., Valencia, Calif., which is shown inFIG. 1.

Referring to FIG. 1, a liquid photopolymer 10 in a vat 12 is positionedbeneath a moveable HeCd laser 14. The SLA part 16 is positioned on anelevator 18. The upper surface 20 of the SLA part 16 is positioned justbelow the top surface 22 of the liquid photopolymer 10 so thatsuccessive layers can be added to the SLA part 16.

To produce a physical part, the SLA receives solid or surface modelgeometry data via a specifically formatted input data file known as anSTL file. The STL file contains a topological representation of the partin terms of many small triangular flat-faced facets whose dimensions andorientation in space are precisely defined. The STL file “virtual” partis then mathematically “sliced” by computer software into very thinhorizontal cross sections or layers. The lowest cross section data issent to a computer-controlled optical scanning system controlling thehelium cadmium (HeCd) laser 14. The laser 14 draws out the shape of thecross section down onto the surface of the vat 12 of photosensitiveliquid resin. Ultraviolet radiation solidifies the resin surfacewherever the laser strikes, thereby precisely transforming the crosssection into a thin solid layer. The process repeats itself, layer bylayer, with each polymerized layer adhering to the layer below it, untila final three-dimensional physical part is produced; this layer-wiseassembly is accomplished on elevator platform 18 within the vat 12 whichis lowered incrementally with the creation of each new layer. Finally,the full part is removed from the liquid vat and exposed to highintensity ultraviolet light to fully cure it and complete thepolymerization process.

The SLA process was originally intended to produce prototypes forconceptual and 3D visualization purposes only. However, users ofstereolithography quickly began to desire to actually test theprototypes in the laboratory. Since the first generationstereolithography polymer resins were typically brittle, low-strength,and prone to warping, second generation epoxy-based photopolymers weredeveloped with improved mechanical properties and dimensional stability.One of these is disclosed in U.S. Pat. No. 5,437,964 to Lapin et al.However, except for very carefully designed experiments as is reported,for example, by W. H. Dornfeld, (1994), “Direct Dynamic Testing ofScaled Stereolithographic Models” International Gas Turbine andAeroengine Congress and Exposition, The Hague, Netherlands (ASMEPrepromt 94-GT-271), the improved polymers to date still have notachieved the mechanical strength necessary for general laboratorytesting loads (e.g., high-speed in-water testing for marineapplications, high-speed centrifugal loading, etc.).

Other prior art related to stereolithography and mixing materials intothe fluid medium used in that process are summarized as follows.

U.S. Pat. No. 5,248,456 to Evans, Jr. et al. discloses an improvedstereolithographic apparatus and method. In one embodiment, theimprovement includes immersing at least a portion of a part in a volumeof a liquid solvent in a vapor degreaser while subjecting the portion toultrasonic agitation to substantially remove excess resin. Severalexamples of solvents are provided, including ethanol, and FREON™. In asecond embodiment, the improvement includes building the part on a layerof liquid resin supported by a volume of a dense, immiscible and UVtransparent intermediate liquid, and integratably immersing at least aportion of the built part in the intermediate liquid, and then eithersubjecting the immersed portion to ultrasonic agitation to substantiallyremove excess resin, or subjecting the immersed portion to UV light.Several examples of intermediate liquids are provided, includingprefluorinated fluids, such as FLUORINER™ FC-40 and water-based saltsolution, such as solution of magnesium sulfate or sodium chloride inwater.

U.S. Pat. No. 5,296,335 to Thomas et al. discloses a method ofmanufacturing a three-dimensional fiber-reinforced part utilizing thesingle-tool method of stereolithography. The tool is fabricated bydesigning the tool on a computer-aided design system and curingsuccessive layers of a fluid medium via a computer-controlledirradiation source to form the three-dimensional tool. The desired partis generated by applying layers of resin-wetted fabric to the tool,curing the fabric on the tool, removing the tool from the designed part,and cleaning, trimming and inspecting the designed part.

U.S. Pat. No. 5,688,464 to Jacobs et al. discloses a method andapparatus for providing a vibrational enhancement to the recoatingprocess in stereolithography. The formation of a thin layer of buildingmaterial over a previous layer of structure of a partially completedthree-dimensional object, in preparation for formation of an additionallayer of structure is enhanced by the use of vibrational energy impartedto the building medium. In a first preferred apparatus, vibration isinduced into the surface of the material by a plurality of vibratingneedles that penetrate below the working surface to a sufficient depthto ensure adequate coupling but not deep enough to come into contactwith the surface of the partially completed part. In a second preferredapparatus, vibration is coupled directly to the object support. Thevibrational energy is then transmitted through the part to the surfaceof the building material. In a first preferred method, the partiallycompleted object is overcoated with material and vibration is used toreduce the coating thickness. In a second preferred method, thepartially completed object is under-coated with material and vibrationis used to increase the coating thickness.

U.S. Pat. No. 5,731,388 to Suzuki et al. discloses photocurable resinscontaining unsaturated urethane of a specified form and vinyl monomerwhich is N-(meth)acryloylmorpholine or its mixture with di-oldi(meth)acrylate at a rate within a specified range and compositionscontaining such a resin and a filler such as solid particles and/orinorganic whiskers of specified kinds at a specified rate are capable ofyielding stereolithographed objects with improved mechanical and thermalproperties and form precision.

U.S. Pat. No. 6,003,832 to Ueno et al. discloses a mold having a cavityfor shaping a three-dimensional object, which comprises a photocuredresin composition including a liquid photocurable resin and at least onereinforcing agent selected form the group consisting of inorganic solidparticles having an average particle diameter of 3 to 70 μm and awhisker having an average diameter of 0.3 to 1.0 μm, a length of 10 to70 μm and an aspect ratio of 10 to 100 and optionally, in which theinner surface of the cavity is covered by a solid film having athickness of 5 to 1000 μm.

Unlike the common method of using the SLA prototype as “wax” masters forinvestment casting of metal parts as described in U.S. Pat. No.4,844,144 to Murphy et al., there have been attempts at strengtheningthe actual SLA prototype itself to allow its direct use in testing. Thesimplest, yet most limited, method is to perform post-stereolithographymilling and drilling operations to allow the insertion of strengtheningagents such as rods, plates, etc. Another option is to modify the SLAoperation in such a way as to allow the insertion of non-polymercomponents (e.g., metal, ceramic) directly during the SLA process suchas in the invention describe in U.S. Pat. No. 5,705,177 to Roufa et al.Another option is the deposition of various metalized coatings to theSLA prototype to both strengthen and protect it for laboratory testingpurposes. Finally, U.S. Pat. No. 5,296,335 to Thomas et al. patented amethod that utilizes stereolithography parts to create a tool and theapplication of resin-wetted fabric on the tool to createfiber-reinforced parts. This patent envisions the removal of thestereolithographic tool but clearly one may leave it inside if necessaryfor support purposes during testing.

While the invention of the newer more capable SLA photopolymersdiscussed above has been helpful in allowing carefully designed testingof SLA prototypes to occur, in general the progress has been slow andlimited. Utilizing even the most advanced photopolymer in commercial usetoday still puts rather severe limitations on available laboratorytesting of SLA prototypes.

The insertion of metal or ceramic structural support members viadrilling and milling operations is only practical for the simplest ofgeometries. In a more complex SLA prototype, it may not even be possibleto utilize this method due to part size, required internal voids in thepart, part slenderness, drastic curves or severe changes in angulardirection, or inability to support the part in a specific requireddirection.

Of the methods currently in use for structural strengthening of SLAprototypes for testing, the incorporation of external coatings discussedoffers the best chance for success. However, even this method is limitedto some degree to fairly simple geometries. For example, it isimpossible to strengthen internal supports with this method. Clearly,this method is not complementary to the very strength of thestereolithography process-namely, the power to generate intricate,highly complex geometries with multiple internal cavities.

It has been well known for many years that the radiation pressure ofacoustic waves may be used to control or manipulate intermittanciese.g., bubbles, particles, etc. in a fluid medium (see for example,Hanson, A. R., E. G. Domich and H. s. Adams, (1964), “Acoustic LiquidDrop Holder”, Rev. Sci. Instrum.,Vol 35, pp. 1031-1034). In fact, thismethod can easily be used to cause fluid motion itself. More recently,arrays of modern acoustic transducers have been employed in moreadvanced ways to move and segregate particles.

U.S. Pat. No. 4,743,361 to Schram discloses a method for separatingparticle types from a mixed population of particles in a liquid. Thisseparation is obtained using an ultrasonic wave produced by interferencebetween the outputs from spaced ultrasonic sources. One or more selectedparticle types may be separated by displacement axially along thestanding wave or transversely through the standing wave or throughcombination of both methods. The described separation can be achieved bycontrol of flow of the liquid or giving the standing wave a drift, or bycontrolling the intensity or the frequency of the standing wave or byany combination of these factors.

U.S. Pat. No. 4,983,189 to Peterson et al. discloses a method andapparatus for controlling the movement of materials having differentphysical properties when one of the materials is a fluid. The inventiondoes not rely on flocculation, sedimentation, centrifugation, thebuoyancy of the materials, or any other gravity dependentcharacteristic, in order to achieve its desired results. The methods ofthe Peterson et al invention provide that a first acoustic wave ispropagated through a vessel containing the materials. A second acousticwave, at a frequency different than the first acoustic wave, is alsopropagated through the vessel so that the two acoustic waves aresuperimposed upon each other. The superimposition of the two wavescreates a beat frequency wave.

U.S. Pat. No. 5,803,270 to Brodeur, discloses accurate ejection ofliquid droplets and agitation of liquids. Oeftering, R. C.,“Manipulation of Liquid by Use of Sound”, NASA Tech Briefs, December,1998, pp. 72-75, describes a very good example of a typical modernacoustic-radiation pressure phased array concept for performing suchoperations. The main benefit of all these acoustic manipulationinventions is their ability to exert control over a fluid medium and/orobjects in the fluid medium without intruding into its container asshown in FIG. 2.

Referring to FIG. 2, a set of left and right phased array transducers 24and 26 are employed to nonintrusively control and manipulate theposition of a dissimilar object 28 in a fluid medium 30 using acousticradiation pressure.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a means ofstructural strengthening of SLA prototypes.

It is another object of the invention to provide structuralstrengthening without interfering with the ability to form complexshapes.

It is yet another object of this invention to strengthen an objectinternally.

Those and other objects are accomplished by the present invention, whichis a method for producing a three-dimensional object by first providinga fluid medium having a top surface and which is capable ofsolidification when subjected to a prescribed stimulation. A solidreinforcing material is then mixed with the fluid medium. Successivecross sectional laminae are then formed, wherein each has a top surfaceof said object at a two-dimensional interface. These cross sectionallaminae are moved downwardly as they are formed, such that there is alayer of the fluid medium between the top surface of the most recentlyformed lamina and the top surface of the fluid medium. The object isbuilt up in step wise fashion so that each lamina is formed from atleast part of the layer of the fluid medium between the top surface ofthe most recently formed lamina and the top surface of the fluid. Asolid reinforcing material is then mixed with the fluid medium so thatat least a part of said solid reinforcing medium is located in the layerof the fluid medium between the top surface of the most recently formedlamina and the top surface of the fluid medium. An acoustic force fieldis then established in the fluid medium. The acoustic force field existsin at least part of the layer of the fluid medium between the topsurface of the most recently formed lamina and the top surface of thefluid medium so that the solid reinforcing material is moved.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome apparent upon reference to the following description of thepreferred embodiments and to the drawing, wherein correspondingreference characters indicate corresponding parts in the drawing andwherein:

FIG. 1 is a schematic cross sectional view of a prior artstereolithography apparatus (SLA);

FIG. 2 is a schematic drawing of a prior art concept for manipulatingparticles in a fluid; and

FIG. 3 is a cut away front elevational view of a system representing apreferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 3 shows an elevational view of the present invention. Astereolithography apparatus (SLA) machine 30 similar to the SLA machineshown in FIG. 1, which has been outfitted with four distributed planaracoustic arrays as, for example, arrays 32 a and 32 b, which consist,for example, of many individually controlled piezoceramic acoustictransducer elements 34 on the interior of each of the vat's fourvertical walls 36. Acoustic arrays 32 a and 32 b are on the opposed sidevertical walls 36 of the vat while two other arrays 32 c and 32 d are onthe opposed end vertical walls 36. The four arrays 32 a 32 b, 32 c and32 d are designed and mounted within the liquid photopolymer bath 38 insuch a way as to not disrupt the workings of the perforated elevatorplatform 40. Additionally, the acoustic arrays 32 a, 32 b, 32 c and 32 dare positioned and oriented so that superimposed acoustic waves 42 maybe generated. These waves 42 overlap in the “thin” layer region 44 ofliquid polymer 38 between the liquid surface 46 and the top portion ofthe solidified SLA part 48 for all vertical positions of the elevatorplatform 40. This relationship is maintained throughout the phases offabrication. As discussed previously, the SLA machine 30 includes alaser 52 and an elevator 40. Laser 30 is joined to laser positionalcontrol equipment 53, and elevator 40 is joined to elevator controlequipment 55. Laser positional control equipment 53 and elevator controlequipment 55 are joined to an SLA machine controller 57. The currentinvention adds an acoustic controller 54 that is joined with SLA machinecontroller 57 for coordinating acoustic signals with the position oflaser 52. Acoustic controller 54 is also attached to each acoustic arrayas, for example, 32 a and 32 b for providing acoustic signals to eachtransducer 34.

The acoustic arrays as, for example, arrays 32 a and 32 b are used tofocus an acoustic beam 42 and thereby apply acoustic radiation pressures(and thus forces) to short whisker-like fibers 50 suspended within theSLA photopolymer bath 38. The superimposed acoustic waves allowmanipulation and control of the positioning of the fibers 50 within thebath. Specifically, it is envisioned that these fibers 50 are directedand their position maintained in the thin layer region 44 of liquidphotopolymer 38 above the solidified part 48 during the laser 52 sweepportion of each SLA layer cycle. Thus, the fibers 50 will automaticallybe entombed in the precise desired positions within the final solidifiedSLA part 48. The precise focusing and positioning of the fibers 50 isaccomplished via appropriately altering the amplitude, phase andfrequency of the individual transducer elements 34 in the acousticarrays, as for example, array 32 a and 32 b using conventional acousticbeamforming practices and acoustic controller 54. In coordination withSLA machine controller 57, acoustic controller 54 can manipulate fibersand particles in many different ways to give desired characteristics. Asingle layer can be provided with a uniform particle size or fiberorientation. Differing fiber orientations allow cross-linkedstrengthening of the object. The point of solidification under the lasercan also be provided with a selected particle size or orientation.

The phased acoustic array beamforming used herein allows concentrationof the fibers 50 in regular bands on a horizontal plane in the thinliquid region 44. The spacing between these rows of high concentrationof fibers is dependent on the instantaneous acoustic wavelength in thephotopolymer bath and can easily be controlled by altering the acoustictransducer operating frequency. The wavelength λ in an acoustic fluid isgoverned by the familiar relation λ=c/f, where c is the speed of soundin the fluid and f is the acoustic wave frequency. Stirring or adding offibers is envisioned throughout the SLA prototyping process in order tokeep their distribution constant.

It is also envisioned that the acoustic properties i.e., mass, densityand acoustic wave speed, of the fibers should be chosen so as to beamenable to acoustic pressure manipulation while being mismatched withthe solidified polymer properties to avoid strongly affecting the solidpart during the SLA process. Furthermore, it is advantageous to choosethe optical properties e.g., wavelength and power, of the laser beam 56and the fibers 50 so that the path of the laser 56 is not greatlyaffected by the presence of the fibers 50. Finally, any resultingsurface deformation caused by the acoustic beam or superimposed acousticwaves can be controlled and limited to workable levels via appropriatemodification of the amplitudes and focusing of the transducers 34.

In addition to obvious gravitational limitations, the size of theobjects, i.e., fiber length, used for the present invention is limitedto some degree by the thickness of the liquid photopolymer layer 44being exposed by the laser on any given sweep. It is possible toincrease the available object size by simply increasing the specifiedlayer thickness during the conventional SLA slicing process. Thismodification is especially appropriate for fabrication of parts withmore simple geometries, where a loss in vertical resolution of the finalSLA part is not overly critical.

The method and system of the present invention provides a means forfabricating whisker fiber-reinforced prototypes directly usingstereolithography. The method and system of the present invention takesadvantage of the nonintrusive nature of acoustic manipulation in a fluidmedium to precisely control the distribution of fibers in a SLAphotopolymer bath during SLA fabrication. For the first time, it ispossible to control the orientation and positioning of fibersinteractively during the entire stereolithography process, ensuring theoptimal distribution and density of fibers throughout the finalsolidified part.

The result is a solidified fibrous composite SLA part with mechanicalstrength sufficient enough to allow actual laboratory testing.Additionally, in contrast to previously mentioned methods for SLA partstrengthening, no post fabrication operations need be performed.Finally, the present invention requires no major modifications toconventional SLA systems and can conceivably be retrofitted to existingsystems.

Versions of the present invention with particles replacing fibers may beconstructed for the creation of particulate composite SLA prototypes.

The proven ability of phased acoustic array systems to segregate andcontrol materials with different physical properties as is disclosed inU.S. Pat. No. 4,743,361 to Schram and U.S. Pat. No. 4,983,189 toPeterson et al. may be exploited to allow the use of both particles andfibers in the present invention for the creation of customizedparticulate/fibrous composite SLA prototypes. It is envisioned that thedistribution of particles and fibers may be controlled duringfabrication to create a solidified composite part with particles incertain desired locations and fibers in others. In fact, with sufficientsignal processing and array geometries, it is even envisioned having amultiple particle sizes and multiple fiber sizes all incorporated into asingle part solidification. A typical fiber that may be used in themethod of this invention is KEVLAR™ which are commercially availablefrom the Dupont Corporation with headquarters at Wilmington, Del.Typical particles that may be used in the method of this invention areglass microspheres, which are commercially available from the 3MCorporation with headquarters at St. Paul, Minn.

While the present invention has been described in connection with thepreferred embodiments of the various figures, it is to be understoodthat other similar embodiments may be used or modifications andadditions may be made to the described embodiment for performing thesame function of the present invention without deviating therefrom.Therefore, the present invention should not be limited to any singleembodiment, but rather construed in breadth and scope in accordance withthe recitation of the appended claims.

What is claimed is:
 1. A system for producing an object comprising: afluid medium having a surface, said fluid medium capable of transformingits physical state in response to a stimulation; a solid reinforcingmaterial provided in said fluid medium; a support means immersed withinsaid fluid medium, and progressively moveable away from said fluidmedium surface; a translational means joined to said support meanscapable of moving said support means with respect to said fluid mediumsurface; a stimulation means capable of providing the stimulationaltering the physical state of said fluid medium at said fluid mediumsurface; at least two acoustic transducer arrays having a plurality oftransducer elements positioned in said fluid medium and capable ofproviding an acoustic field at said fluid medium surface formanipulating said reinforcing material; and an acoustic controllerjoined to said at least two acoustic transducer arrays to control thetransducer elements and the acoustic field by beamforming acousticradiation from the transducer elements.
 2. The system of claim 1 furthercomprising an object controller joined to said translational means andsaid stimulation means, said object controller being capable ofpositioning said stimulation means and said translational means forcontrolling positioning of the stimulation means with respect to thesupport means.
 3. A system for producing an object comprising: a fluidmedium having a surface, said fluid medium capable of transforming itsphysical state in response to a stimulation; a solid reinforcingmaterial provided in said fluid medium; a support means immersed withinsaid fluid medium, and progressively moveable away from said fluidmedium surface; a translational means joined to said support meanscapable of moving said support means with respect to said fluid mediumsurface; a stimulation means capable of providing the stimulationaltering the physical state of said fluid medium at said fluid mediumsurface; at least two acoustic transducers positioned in said fluidmedium and capable of providing an acoustic field at said fluid mediumsurface for manipulating said reinforcing material; an acousticcontroller joined to said at least two acoustic transducers forcontrolling the provided acoustic field; and an object controller joinedto said translational means and said stimulation means, said objectcontroller being capable of positioning said stimulation means and saidtranslational means for controlling positioning of the stimulation meanswith respect to the support means; wherein said object controller isjoined to said acoustic controller for coordinating the position of theprovided acoustic field with the portion of the fluid medium beingsubjected to said stimulation means.
 4. The system of claim 3 furthercomprising a vat having a plurality of walls containing said fluidmedium therein, said acoustic transducers being positioned on at leasttwo of said walls.
 5. The system of claim 1 wherein the solidreinforcing material is selected from a group consisting of a fibrousmaterial, a nonfibrous material and a mixture of a fibrous material anda nonfibrous material.